Methods For Dual-Scale Surface Texturing

ABSTRACT

Methods for preparing a substrate surface are provided, for purposes including manufacturing a low reflectivity surface. In some aspects, the methods include providing a material comprising an etching mask on a substrate, subjecting the material to a first isotropic etching phase, and subjecting the material to a first anisotropic etching phase, thereby forming a textured surface on the material, wherein the textured surface comprises structures with dimensions in a sub-micron range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the U.S. Provisional PatentApplication No. 61/680,611 filed on Jul. 31, 2013 and entitled“DUAL-SCALE SURFACE TEXTURING”, the entire disclosure of which is herebyincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under 1041895 awardedfrom the National Science Foundation and the Department of Energy. TheUnited States government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The field of the invention relates to a process and method for preparingat least one surface of a substrate. In one embodiment, the inventionrelates to a dual-scale surface texturing method involving isotropic andanisotropic etching to provide nanometer-scale and micrometer-scalefeatures capable of determining optical properties of one or moresubstrate surfaces.

Surface texturing is one of several methods utilized across multipleindustries in fabrication processes associated with electronic andoptical devices, biochips, biosensors, and so forth. For example,surface texturing approaches have found use in the manufacturing ofphotovoltaic (solar) cells. In many optical device applications, andother applications, the geometrical aspects, as well as the opticalproperties, of materials employed are used as a way to control theoptical behavior. Specifically, particular textures or structures areutilized to dictate light absorption and reflection characteristics ofdevices. For instance, micrometer- and nanometer-scale featuresassociated with particular solar cell layers can improve energy captureby reducing losses due to reflected light from cell surfaces, andincreasing absorption. In this manner, the efficiency of solar cells canbe increased. In addition, improved capture of incident radiation, byway of a smaller active layer volume required for a desired efficiency,results in a reduced amount of materials needed in the fabricationprocess.

Current surface texturing technologies are limited since they createtexture structures with large sizes, on the order of 5-10 micrometers,and high aspect ratios. For solar cell applications, such featuresproduce significant light reflection, and hence reduce cell efficiency.Hence, there is a need for improved device patterning techniques relatedto producing structures at the nanometer (i.e. sub-micrometer) range.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks byproviding a methodology for producing textured substrate surfaces.Specifically, a method is provided for preparing an etching mask on asubstrate, and utilizing the etching mask to produce textured surfaces.The method also includes performing a dual-scale etching process, usinga prepared etching mask, to achieve a texturing of the one or moresubstrate surfaces.

In one aspect of the present invention, a method of preparing asubstrate surface is provided. The method includes providing a materialcomprising an etching mask on a substrate. The method also includessubjecting the material to a first isotropic etching phase, andsubjecting the material to a first anisotropic etching phase, therebyforming a textured surface on the material, wherein the textured surfacecomprises structures with dimensions in a sub-micron range.

In another aspect of the present invention, a method of preparing asubstrate surface is provided. The method includes dispersing aplurality of particles in a suspending medium to form a suspension, andspin-coating the suspension on a substrate comprising a material to forman etching mask on the substrate. The method also includes subjectingthe material to a first isotropic etching phase, using the etching mask,forming a modified etching mask, and subjecting the material to a firstanisotropic etching phase, using the modified etching mask, therebyforming a textured surface on the material, wherein the textured surfacecomprises structures with dimensions in a sub-micron range.

The foregoing and other aspects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings which form a part hereof, and in whichthere is shown by way of illustration a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference is made therefore to the claimsand herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a silica micro-sphere (SMS) dispersibility comparisonbetween N,N-dimethyl-formamide (DMF) and water, wherein (a) calculatedand measured absorbance is plotted versus wavelength for a single 310 nmSMS in each solvent; and scanning electron microscopy (SEM) images of(b), and (c) show SMS assemblies using DMF and water, respectively.

FIG. 2 shows a SEM image illustrative of the SMS cluster effectproducing non-uniform SMS distribution during spin-coating.

FIG. 3 shows: a contact angle measurement of (a) water and (b) DMF; acomparison of surface coverage with 300 ul (100 mg/ml) for (c)SMS_(water) and (b) SMS_(DMF) solution droplets on 2-inch Si substrate;and surface images of (e) SMS_(water), and (f) SMS_(DMF).

FIG. 4 shows SEM images comparing the coverage difference between (a)SMS_(DMF) and (b) SMS_(water).

FIG. 5 is a schematic illustration of two spheres partially immersed ina fluid layer for capillary attraction, F_(cap).

FIG. 6 is a schematic illustration of (a) fast solvent evaporation, and(b) slow solvent evaporation during spin-coating, producing (c)localized SMS assembly with discontinuous F_(cap), and (d) long-rangeSMS assembly after expanded F_(cap).

FIG. 7 shows SEM images for SMS coverage from various concentration ofSMS_(DMF), which are (a) 50 mg/ml, (b) 100 mg/ml, and (c) 150 mg/ml.

FIG. 8 shows SEM images illustrative of the acceleration rate effect foruniform SMS assembly layer formation; (a) 20 rpm/s, (b) 50 rpm/s, and(c) 80 rpm/s.

FIG. 9 shows a surface image of (a) SMS deposited on a 2-inch round Sisubstrate, and the corresponding SEM images at different magnificationsof (b) 250×, (c) 2000×, and (d) 25000×, respectively.

FIG. 10 shows a surface image of (a) SMS deposited on a 4-inch round Sisubstrate, and the corresponding SEM image at magnification (b) 100×;SEM images (c) and (d) represent regions of low and high SMS monolayercoverage, respectively.

FIG. 11 shows a schematic illustration of RIE process to etch solidsurface.

FIG. 12 shows a schematic illustration and SEM images to fabricatevarious shape of silicon nanostructure with an RIE process.

FIG. 13 shows reflectance measurements for various RIE processed siliconsurfaces.

DETAILED DESCRIPTION OF THE INVENTION

Production of modern-era devices, with increased complexity andever-decreasing dimensions, has required overcoming many technicalchallenges associated with approaching fundamental physical limits. Inlooking to push limits further, there are remaining challenges forproducing devices structures with dimensions in a micrometer andnanometer range at lower-cost and with high-throughput.

The present disclosure describes an approach for treating or processingsubstrate surfaces and materials therein for purposes of producingstructures or features with dimensions in range including a sub-micronrange. As will be described, in some aspects, a substrate can beprepared with single or multiple layers of particles, dispersed and/orself-assembled as desired on a substrate surface. The particles, inaddition to other materials and compositions, may then serve as a maskor protective layer in a surface treatment process for shaping,texturing, or profiling a substrate surface or multiple substratesurfaces.

Among other applications, the present invention can be utilized in thefabrication process of photovoltaic cells. As such, the scale ofcolloidal particles have found important uses in device fabrication, andparticularly in photovoltaic cell manufacture, where the need forincreased cell efficiency and reduced cost has pushed the technologytoward reduced dimensionality and enhanced energy absorption.

In accordance with one embodiment of the present invention, an approachfor achieving a desired layer uniformity using a spin-coating process isdescribed. Particularly when colloidal particles are utilized,uniformity over large substrate areas may be achieved by using a mediumfor suspending the particles with properties configured for efficientself-assembly during a spin-coating process. For example, as will bedescribed, by utilizing a suspending medium with a relatively slow rateof evaporation, as compared to say water, a more consistent convectiveflux of the particles and medium, the particles dispersed or suspendedtherein, can be achieved during a spin-coating process. Similarly, byutilizing a suspending medium in which the particles are evenly oruniformly dispersed before a spin-coating process begins, a uniformityof a resultant layer may be further improved. For example, an aproticmedium for suspending colloidal particles may be used, since such mediumcan possesses characteristics that improve the effectiveness of thespin-coating process.

Particles for use in accordance with aspects of the present disclosure,may be generally sized to an average dimension in the range of 10nanometers to 10 micrometers, and shaped to be spherical, ellipsoidal,and the like, although other shapes and sizes may be possible.Particularly, the average dimension typically refers to the longestlongitudinal length common to the plurality of particles. The particlesmay be manufactured using suitable materials and methods, in accordancewith a desired application. By way of non-limiting example, theparticles may comprise metal oxides, or silica, or alumina, or zirconia,or titania, or carbon, or any combination thereof. The particles may bedispersed in a suspending medium, using suitable techniques, such assonication, to form a suspension with properties according to a desiredconcentration, or other criteria. For example, the concentration of theplurality of particles in the suspending medium can be in the range of0.1 to 10.0 wt %, although other values are possible.

By way of non-limiting example, a suspending medium, for use inaccordance with the present disclosure, can be an amide solvent, or ahydrophilic solvent, or a suspending medium selected from a groupconsisting of tetrahydrofuran, ethyl acetate, acetone,dimethylformamide, acetonitrile, dimethyl sulfoxide, and mixturesthereof. In some aspects of the invention, the suspending medium doesnot include a surfactant. In other aspects of the invention, thesuspending medium may be dimethyl-formamide. In addition, the suspendingmedium can have a boiling temperature in the range of 100 degreesCelsius to 200 degrees Celsius, or a viscosity in the range of to 0.1mPa sec to 1.0 mPa sec at 25 degrees Celsius, or a surface tension inthe range of 20 mN/m to 40 mN/m at 25 degrees Celsius, or combinationsthereof, although other values are possible. As will become apparent,suspending media, with properties as described, facilitatedispersibility, wettability, and evaporation requirements for obtainingan enhanced uniformity of particles on a substrate.

The suspension, or solution containing the particles dispersed in thesuspending medium, can then be distributed onto a substrate in anymanner using, for example, a spin-coating process. In some aspects, thesuspension can undergo a rotational acceleration to a rotational targetspeed between 1000 and 5000 rotations per minute, wherein the rotationalacceleration can be in the range of 50 rotations per minute per secondto 100 rotations per minute per second, although other rotationalacceleration and speed values, along with multiple rotational stages anddurations, may be possible.

In addition, the substrate may be of any type, and can include materialssuch as doped or undoped silicon, although it may be appreciated thatother substrate types and substrate materials may also be possible. Byway of example, the substrate may have a surface area of about 1 cm² toabout 1000 cm², although other values are possible. Moreover, in certainaspects, the dispersed particles form a two-dimensional (“2D”)self-assembled layer on a substrate surface. In other aspects, amonolayer coverage of the particles on the substrate can be in the rangeof 80% to 100% of the surface of the substrate, although other valuesmay be possible.

In accordance with another embodiment of the present invention, a methodfor preparing a substrate surface is provided. As will be described,multiple surface treatments can be utilized to form and shape structuresor features with dimensions in a micrometer and nanometer scale. In someaspects, described treatments may be applied for purposes of generatingtextured surfaces.

Producing structures or features on a substrate surface with dimensionsin a range including a sub-micron range may involve additive and/orsubtractive surface treatments using a number of masking layers. Forinstance, a surface treatment can include an etching process, such as aplasma etching, a reactive ion etching, an ion milling, or any otherphysical or chemical etching process, or material removal process.Additionally, forming desirable shapes and profiles for particularstructures or features may involve use of isotropic and/or anisotropictreatment techniques. Particularly, an isotropic etching process wouldbe capable of removing a material uniformly, while an anisotropicetching process would remove material preferentially along a specificdirection.

Hence, in accordance with aspects of the present invention, preparing asubstrate surface can include subjecting the substrate surface and/ormaterials therein or thereupon to a number of isotropic or anisotropicetching steps, or combinations thereof, for purposes includinggenerating textured surfaces. For example, an isotropic etching can beachieved during a reactive ion etching phase via plasma generated, forexample, using trifluoromethane, or a noble gas, or a sulfur halide or,more specifically, sulfur hexafluoride. The plasma may also includeoxygen. In addition, an anisotropic etching can be achieved by way ofplasma generated using an elemental halogen such as, for example,diatomic chlorine. By way of example, a substrate surface including anetching mask can be subjected to an isotropic etching phase andsubsequently subjected to an anisotropic etching phase. However, it maybe appreciated that other isotropic or anisotropic etching phases arepossible during a surface treatment, and can further be combined orinterleaved with other processes or treatments, including materialdeposition processes. In some aspects, etching processes can partiallyor entirely remove masking layers or materials comprising a substratesurface.

The size, self-assembly, and etch properties of colloidal particles lendthemselves well to substrate surface shaping and profiling at themicrometer and nanometer scale. As such, colloidal particles, shaped,dimensioned, and assembled in accordance with methods described by thepresent disclosure, may be utilized as masking layers to createstructures, features or textures on a substrate surface. For example, amonolayer of silica nanospheres may be applied to a substrate surfaceprior to a surface treatment process. In this manner, colloidal particleassemblies can be used as etching, or deposition, masks to generatestructures, features or textures with sizes in a range between 10nanometers and 10 micrometers, although other sizes may be possible.However, it may be appreciated that other assemblies, materials, layers,and so on, suitably shaped and dimensioned, may also be utilized asmasks. Additionally, one or more masking layers may be applied to thesurface of a substrate. Moreover, any of the above-mentioned maskinglayers can be applied to the entirety of a selected surface, or can beapplied to only a portion of the surface. As described, in some aspects,dispersed colloidal particles forming a 2D self-assembled monolayer withcoverage in the range of 80% to 100% of the surface of the substrate canbe utilized.

It is contemplated that any combination of micro-scale and nano-scaletextures may be produced using methods described, to include structuresand features of any shape, size and pattern. By way of example, suchstructures and features can be selected from or include one or more ofrods, cones, frustums, pyramids and needles. In some aspects, nano-scalestructures may be disposed on micro-scale structures, to include one ormore of rods, cones, frustums, pyramids and needles. For example, insome forms, a textured substrate surface may have spiked nano-rodstructures. In addition, the textured surface may further include asubstantially uniform array or a substantially ordered array. Thetextured surface may further include silicon nano-structures.

With respect to solar cell applications, the above methods can beapplied to produce particular features or structures advantageous ingenerating a low reflectivity surface. For example, a textured surfacemay be formed to exhibit a weighted reflectance of less than 5% over awavelength range of about 300 nanometers to about 1100 nanometers. Inthis manner, such textured surface can provide both light trapping forenhanced cell efficiency and anti-reflection coating effects. Using adual-scale texturing process, large scale (micron-scale), and smallscale (sub-micron) features may be created in a substrate surface,whereby the micro-scale textures can increase the light path byincreasing the number of surface reflections, while the sub-micron scaletextures provide a gradient refractive-index at the interface producingan anti-reflection coating effect.

Specific examples are provided below. These example are offered forillustrative purposes only, and are not intended to limit the scope ofthe present invention in any way. Indeed, various modifications of theinvention in addition to those shown and described herein will becomeapparent to those skilled in the art from the foregoing description andthe following example and fall within the scope of the appended claims.

EXAMPLE I

Polished n-type Si (100) round substrates, 2-inch and 4-inch in diameterwith 280 μm and 460 μm thickness, respectively, were used to demonstratethe dispersibility of particles on a substrate. As a preparation step,the substrates were cleaned in piranha solution [H₂SO₄ (96%): H₂O₂(30%)=4:1] for 15-min to form hydrophilic Si surface followed by 10-minde-ionized water (DI-water) rinse.

Next, silica micro-sphere (SMS) solutions for spin-coating were preparedby adding 310 nm-diameter silica microsphere powder to separatesolvents, namely N,N-dimethyl-formamide (DMF) and de-ionized water. Thesolutions were subjected to a sonication for 5 hours to agitate theparticles in order to produce complete dispersion of SMS in thesolution. The SMS dispersion for each solvent was then characterized bymeasuring absorbance using ultraviolet-visible (UV-VIS)spectrophotometer. Subsequently, the solutions were spin-coated onto theSi substrates, using a Delta80BN spinner (SUSS MicroTec), under ambientlaboratory conditions (21˜23° C. temperature and 25˜30% humidity). Asingle step of spin-coating recipe was implemented using variousacceleration rates (20-80 rpm/s) for 2000 rpm target speed. The SMSspin-coated substrates were subsequently examined by scanning electronmicroscopy (SEM, JEOL XL-30), and coverage of SMS was calculated usingimage analysis software, “Image J” (National Institutes of Health, USA)after the direct counting of the SMS area. The wettability of solvent onSi wafer was assessed by measuring contact angle with EasyDrop contactangle measurement system (KRUSS).

To demonstrate the excellence of DMF for 2D colloidal particlespin-coating on large-scale surface area, one of the most widely usedsolvent, water, has been selected to compare the quality of SMSmonolayer assembly layer, and the coverage after spin-coating underambient conditions.

Dispersibility in Solution

Before performing a spin-coating of particles dispensed in a dispersingmedium for use in, for example a lithographical process, a uniformdispersion of particles in the medium may be advantageous to avoidclustered deposition on a substrate surface. Therefore the choice ofmedium, such as N,N-dimethyl-formamide (DMF), helps to ensure a highdispersibility.

Turning to FIG. 1( a) a plot of absorbance versus wavelength is shownfor 0.5 wt % 310 nm SMS dispersed in DMF (denoted as SMS_(DMF)) andwater (denoted as SMS_(water)). Using a UV-VIS spectrophotometer,normalized absorbance measurements (solid lines) were performed tocompare SMS dispersibility for each solvent. Also, extinctioncross-sections calculations for a single SMS of the same size (dashedlines) were fitted to the experimental measurements (solid lines). Asshown, DMF produces a high level of SMS dispersibility since themeasured absorbance (denoted as DMF_(UV-VIS)) of SMS in DMF has a verywell-matched trend with the calculated extinction cross-section (denotedas DMF_(calc)), indicating SMS_(DMF) was nearly completely dispersed. Bycontrast, for SMS_(water) the experimental absorbance (denoted asWATER_(UV-VIS)) was substantially broadened compared to calculatedextinction cross-section (denoted as WATER_(calc)). This spectralbroadening of absorbance suggests that particles larger than 310 nm aredispersed in the water solution, which appears to result from aggregatedSMS. FIG. 1( b) and FIG. 1( c) highlight the dispersivity differencebetween spin-coated solutions of SMS_(DMF) and SMS_(water), wherein FIG.1( b) shows the appearance of clusters on the substrate surface.

The existence of a large number of SMS clusters in a dispensed solutionhelps to produce uniform monolayer deposition during spin-coating. Dueto a heavier weight, and hence higher surface friction, clusters cananchor to the substrate surface and consequently act as a flow barrier,preventing uniform distribution of the SMS. This is illustrated in FIG.2, in which poor SMS dispersibility in a solvent is shown to produce alarge, SMS-free area a radial direction induced by the spinning process,and thus significantly affecting the coverage of SMS monolayer.Therefore, a high level of SMS dispersibility in solution is desired toproduce a high uniformity and coverage of SMS monolayer followingspin-coating.

Wettability on Silicon Substrates

Turning to FIG. 3, the contact angles (a) of DMF and water on Si surfacewere measured to compare the degree of wettability of each solvent.Comparing FIGS. 3( a) and 3(b), DMF is shown to offer an outstandingwettability [α_(DMF)≈0 in FIG. 3( b)] compared to water [α_(water)=26.9°in FIG. 3( a)] on the piranha cleaned Si surface. The importance ofsolvent wettability is a crucial factor to obtain highly uniformmonolayer coverage, the reason being that a high wettability can providefast, uniform, and omni-directional spread-out during (or even before)the spin-coating step due to a low surface tension. For a wettabilitycomparison between DMF and water, 300 ul volume solutions of 100 mg/mlSMS_(water) and SMS_(DMF) were been dispensed on piranha-cleaned 2-inchSi substrates. From FIGS. 3( c) and 3(d), it is clear that the SMS_(DMF)solution shows a complete wetting layer on the surface once the solutionwas dropped, whereas the same solution volume of SMS_(water) producedonly a partially-covered surface.

The significance of wettability for spin-coating is illustrated in FIGS.3( e) and 3(f), where the surface uniformity of SMS_(water) andSMS_(DMF) after spin-coating is illustrated. The images clearlyillustrate that, unlike water-based solution, a DMF-based solution canproduce outstanding uniformity from the center of the substrate all theway to the edge. The significantly improved uniformity can be explainedby the low surface tension (γ) of DMF (γ_(DMF)=25 mN/m) as compared thatof water (γ_(water)=73 mN/m). For high γ solvent (e.g. water), a strongcentrifugal force improves solution distribution. However, a strongcentrifugal force is achieved by a high rotation speed, which in turnwill also cause fast solvent evaporation. A fast evaporation can thenremove a significant solvent volume before a uniform SMS distributioncan made. As such, formation of non-uniform SMS layer is unavoidable,and thus a low γ solvent is more desirable for uniform SMS assembly.

In addition, a low γ is also beneficial to producing a large SMScoverage. This is because a lower γ solvent, utilizing slower spinningspeeds, produces only small amounts of SMS loss in the spinning process.By contrast, a high γ solvent produces a large loss of SMS due to thestrong centrifugal force to distribute the solution during spin-coating.This difference is illustrated in FIG. 4, wherein SEM images reveal avery noticeable coverage improvement of SMS_(DMF) over SMS_(water). Forthis example, the images were obtained at the center of each substrateto exclude possible secondary SMS delivery during solution spreading.Therefore, it is clear that the great wettability of DMF can offerexcellent uniformity in the assembled SMS layer, with outstandingcoverage and small loss of SMS.

Solvent Evaporation Rate

For highly uniform and closely-packed SMS monolayer formation using aspin-coating approach, two processes should be efficiently taken intoaccount, namely the (1) capillary assembly, and (2) convective flux.Turning to FIG. 5, capillary assembly, which uses capillary forces toorganize particles in a suspension, is dominant at the short rangebecause the magnitude of capillary force is inversely proportional tothe inter-particle distance, as follows:

F _(cap)=2πγr _(c) ²(sin²Ψ_(c))(1/L)  (1)

where γ is the surface tension of a liquid, r_(c) is the radius of thethree-phase contact line at the particle surface, Ψ_(C) is the meanmeniscus slope angle at the contact line, and L is the distance betweenthe centers of the particles as is illustrated in FIG. 5

To generate a close-packed SMS monolayer over an expanded surface area,a sufficient particle flux provides for uninterrupted growth of SMSassembly monolayer. Therefore, an effective SMS flux should followinitial nucleation of SMS monolayer caused by an initial capillaryassembly. For a spin-coating process, this SMS flux may be convective,which originates from the different hydrodynamic forces induced by thevariation in wetting layer thickness from substrate center to edge whilespinning.

The evaporation of solvent induces a gradual decrease of the wettinglayer thickness with time, and until the wetting layer is thicker thanthe SMS diameter, it decreases evenly over wetting area. Once orderedregions are formed, there would be solvent convective flux from thickerwetting (or disordered) region to thinner wetting (or ordered) regionfollowed by SMS flux. During this convective flux, there is a differentevaporation rate between ordered and disordered regions due to theslower solvent evaporation rate in the ordered region caused byhydrophilic property of the SMS. With water, however, its high vaporpressure (VP_(water)=17.54 Torr at 20° C.) leads a rapid evaporationrate at disordered region during spin-coating, and produces fastreduction of fluid level variation from ordered region to disorderedregion.

This process is illustrated in FIG. 6. For a solution with a fastevaporation rate, convective flux may only occur for short period oftime. As such, an insufficient convective flux as illustrated in FIG. 6(a) leads to only a short-range SMS ordered region, formed through alocalized F_(cap), as shown in FIG. 6( c). Consequently, in order toachieve long-range SMS assembly, solvents with high vapor pressure (e.g.water, methanol) have required additional treatment involvingsurfactants or have necessitated systems for temperature and humiditycontrol in order to provide delayed evaporation.

By contrast, DMF, has a slow evaporation rate caused by its low vaporpressure (VP_(DMF)=2.7 Torr at 20° C.), which can then lead to the longperiod convective flux needed to deliver sufficient amount of SMS fromdisordered region to ordered region, shown in FIG. 6( b). Consequently,a long range SMS assembled region can be achieved, with an expandedF_(cap) on the substrate surface, shown in FIG. 6( d). The slowevaporation rate of DMF, combined with excellent dispersibility andenhanced wetting properties have successfully produced well close-packedSMS monolayer assembly with an outstanding coverage on the surface, aswill be shown in following section.

Large-Scale Area SMS Monolayer Spin-Coating with DMF

In addition to the choice of solvent, the solution concentration andspin-coating speeds are also effective parameters in assembling highlyuniform SMS monolayers with great coverage on large-scale surface area.Turning to FIG. 7, the effect of SMS_(DMF) concentration on substratecoverage is illustrated. The figure shows SEM images of spin-coatedsubstrates using 50 mg/ml, 100 mg/ml, and 150 mg/ml of SMS_(DMF),without a spin-coating process optimization, namely 20 rpm/sacceleration, 2000 rpm for 150 sec. It can be seen that a higher SMScoverage is achieved with increased concentration, and at 150 mg/mlSMS_(DMF), a complete surface coverage was formed after spin-coating.However, with increased concentrations of SMS_(DMF) a more severenon-uniformity of SMS assembly layer is observed, which may be due tothe increased viscosity of solution at higher SMS concentrations.Therefore, a well-optimized spin-coating process is necessary foruniform distribution of SMS over the substrate surface.

Previously, for self-assembled microsphere (MS) monolayer deposition,conventional spin-coating processes involve two steps: (1) a dispersionstep at slow speed rotation for uniform MS distribution on the surface,and (2) a drying step at high speed rotation for removing solventresidue and prevent further solvent interaction with MS afterspin-coating. This two-step spin-coating process has been developedbecause the conventional solvents, like water, have a high γ thatproduce a large loss of MS for high spinning speeds. Moreover, fastevaporation rates at high speeds prevented the manufacture of uniform MSassembly. As such, a slow spin speed dispersion step is needed forincreased uniformity with reduced MS loss.

However, with DMF only a one step spin-coating process may be sufficientdue to the great wettability and slow evaporation rate. In the exampleprocess, target speed is fixed at 2000 rpm, and only acceleration ratesare changed, for a total of 150 sec spin-coating duration. Turning toFIG. 8, the effect of acceleration on the formation of a SMS assemblylayer is shown. The figure shows SEM figures of coverage obtained usingaccelerations of 20 rpm/s, 50 rpm/s, and 80 rpm/s. As the accelerationrate is increased, an enhanced surface morphology of SMS assembly layersis observed, whereby at 80 rpm/s acceleration a uniform SMS monolayer isachieved.

Turning to FIG. 9( a), the image of 2-inch substrate is shown afterundergoing spin-coating with 80 rpm/s acceleration. Aside frominsignificant surface defects, examining the SMS monolayer at highmagnification in FIG. 9( d), the overall uniformity is excellent.Moreover, more than 95% of average coverage has been achieved, which isthe highest SMS monolayer coverage on 2-inch substrates ever reported byspin-coating.

Turning now to FIG. 10, in order to explore the feasibility of DMF forSMS monolayer assembly on even larger-scale, 4-inch round Siliconsubstrates were prepared. An identical spin-coating recipe and SMS_(DMF)concentration was applied to examine the area dependence, and only thesolution volume was adjusted to 800 μl to account for the increasedsurface area. The figure demonstrates a great overall coverage of SMSmonolayer on the 4-inch substrate, with great uniformity from center toedge. Although FIG. 10( c) identifies a few areas with relatively lowSMS coverage, consideration should be made that the process has not beenoptimized for 4-in area deposition. As such these issues may be resolvedby further adjustment of spin-coating process or solution concentration.Nevertheless, this process provides more than 90% of average monolayercoverage, which is still a superb SMS coverage assembled by aspin-coating process. In addition, it should be highlighted that theseresults were achieved by spin-coating under common ambient laboratoryconditions, without any surfactant mixture or additional treatment onthe substrate and SMS. Therefore, the unique solvent properties of DMFare believed to offer a high tolerance to large-scale surface area SMSspin-coating accompanied by outstanding SMS monolayer uniformity andcoverage.

In the present invention, we introduced a new organic solvent,N,N-dimethyl-formamide (DMF) for silica microsphere (SMS) monolayerspin-coating on Si surface, which has proven its great potential forhigh-throughput spin-coating process application leading large-scalearea coverage of well close-packed SMS monolayer assembly without anysurfactant mixture and environment control during spin-coating. Weshowed that the DMF can provide outstanding competence to replaceconventional solvents, (e.g. water, and methanol) to enhance theuniformity, coverage, and packing of SMS monolayer even under theambient laboratory spin-coating environment.

From a comparison with water, DMR was shown to offer enhanced propertiesfor spin-coating applications. We demonstrated that DMF allows forwell-dispersed SMS in the medium that is close the theoretical limit,which is an important property in producing a uniform SMS distributionon the surface of a substrate. Moreover, the outstanding wettability ofDMF forming a thin wetting layer on the substrate surface providessuperb coverage of SMS assembly layer compared to same volume andconcentration of SMS in water. As such, we have successfully shown morethan 90% of SMS monolayer assembly on 2-inch (˜95%) and 4-inch (˜90%) Sisubstrates without the need for additional surfactant additives norspecial environment control (humidity and temperature) for spin-coating.Therefore, it is clear that DMF offers a great potential forhigh-throughput, easy, and low-cost spin-coating process to producehighly uniform 2D colloidal particle assembly on large-scale depositionarea.

EXAMPLE II

Plasma-assisted RIE combines physical and chemical etching which can bedone by ion sputtering, and chemical reaction of radicals with targetmaterials. RIE uses a gas glow discharge to dissociate and ionizerelatively stable molecules, thereby forming chemically reactive andionic species. The etching chemistry is formulated such that thereactive and ionic species formed react with the solid surface to beetched to form volatile products.

The processes taking place during RIE process are described below andschematically illustrated in FIG. 11. The key terms used in describingRIE are as follows:

Generation—A glow discharge is used to generate the gas phase etchingenvironment. The gas phase is generated from a suitable feed gas (e.g.,SF₆ for silicon etching) by electron-impact dissociation/ionization. Theresulting gas phase etching environment consists of neutrals, electrons,photons, radicals (e.g., F*) and positive (e.g., SF⁺⁵) and negative(e.g., F⁻) ions.

D.C. bias formation—A substrate, such as a silicon wafer is placed on aradio-frequency (RF) driven capacitatively-coupled electrode. Theelectron mobility is much greater than the ion mobility. Therefore,after ignition of the plasma, the electrode acquires a negative charge,which is the direct current (D.C.) self-bias voltage.

Diffusion/forced convection—The transport of reactive intermediates fromthe bulk of the plasma to the silicon surface occurs by diffusion.Positive ions from the glow region are forced to the substrate surfaceby way of the D.C. self-bias (negative) voltage and will assist in theetching reaction.

Adsorption—Reactive radicals adsorb on the silicon surface. This stepcan be strongly enhanced by concurrent ion bombardment which serves toproduce active sites as it aids in the removal of, for example, theSiF_(x) layer which otherwise passivates the Si surface.

Reaction—The desired reaction occurs between the adsorbed species andthe silicon. In the case of fluorine-based etching of silicon, chemicalreactions between fluorine atoms and the surface spontaneously produceseither volatile species, such as SiF₄, or their precursors, such asSiF_(x), where x<4. However, in chlorine-based etching, atoms adsorbreadily on silicon surfaces, although the spontaneous etch rate is veryslow. Ion bombardment makes it possible for adsorbed chlorine atoms toattack the backbones of silicon more efficiently and form a volatileSiCl₄ molecule. This mechanism is called ion-induced RIE.

Desorption—The desorption of the reaction product into the gas phaseentails that the reaction product is volatile. Thus, it should have ahigh vapor pressure at the substrate temperature. Additionally, thereshould be no deposited blocking film at the surface. The removal ofthese films can be greatly accelerated by ion bombardment viasputtering. This mechanism is known as ion-inhibitor RIE.

Exhaust—The desorbed species diffuse from the etching surface into thebulk of the plasma and should be pumped out. Otherwise, plasma-induceddissociation of product molecules will occur and re-deposition can takeplace.

In certain embodiments, prior to etching, a masking layer is applied toa surface of the substrate. This mask can be applied to the entirety ofthe selected surface, or it can be applied to only a portion of thesurface. In one embodiment, silica nanospheres are applied to thesurface. The application of the nanospheres can be accomplished usingthe solvent-controlled spin coating techniques described herein such asin Example I. Based on experimental results, a silica NS solutionincluding DMF provided greater control over NS deposition due to theunique properties of DMF including extremely low contact resistance tothe silicon surface and relatively high boiling temperature (b.p.=153°C.) as compared with a conventional solvent such as water.

Silicon surface etching was achieved with using chlorine- andfluorine-containing gases. The use of these gases in RIE provides morecontrol over the surface texturing process. Two categories of RIEinclude isotropic and anisotropic etching. An isotropic etching processinvolves the non-directional removal of material from a substrate,whereas anisotropic etching is a more defined, directional etchingprocess. For example, anisotropic etching can involve primarily verticaletching in one-dimension. By comparison, isotropic etching can involvenon-specific etching from multiple directions. Fluorine-based plasmasoffer high reaction probability with silicon, which is suitable forisotropic silicon etching. In contrast, low reaction probability ofchlorine can offer a highly anisotropic etching environment producingsurface structures with greater profile control. Therefore, bycombination of fluorine and chlorine based RIE processes under variousetching conditions (for example, gas flow rate, RF power, pressure) adiversity of nanostructures can be fabricated with relatively easycontrol.

Referring to FIG. 12, the RIE texturing process has been illustratedwith actual SEM images provided for each step. An exemplary substrate(a) including a nanosphere monolayer on a surface of the substrate canbe a starting substrate. The first etching step can use an etching gas,such as CHF₃/Ar, to reduce the size of the silicon microspheres (SMS)(SiO₂), thereby forming substrate (b). This step provides an etchingpath for the following RIE processes. Concomitantly, this stepdetermines the lateral size of the nanostructures. After SMS sizereduction, Cl₂ (10 sccm & 30 mTorr) etching is used to form nanopillarstructures by intensified anisotropic etching with high RF power (100watts) as shown for substrate (c). Under continuous etching, however,the increased interparticle distance between SMS on top of siliconnanopillar accelerates the etching rate of silicon from the top down.This results in weakened anisotropic etching, thereby producing a variedetching rate of silicon from top to bottom as for substrates (d) and(e). With extended etching, the SMS can be completely removed and thetop of silicon anisotropically etched to produce dual-scalenanostructure as shown for substrate (h). If reduced RF power (50 watts)is applied under the same conditions, then the overall density andenergy of the free electrons is decreased. This produces a less negativeD.C. voltage causing further weakened anisotropicity and etching rate.Consequently, a super-sharp nano-tip structure can be fabricated asshown for substrate (f). However, a low-aspect ratio nanotip structurecan be achieved as in substrate (g) by switching from, for example, aCl₂ to an SF₆/O₂ based plasma in order to provide intensified isotropicetching. It is noteworthy that SF₆ etching can occur rapidly, soprocesses incorporating this chemistry can be operated for a shorterperiod to prevent over-etching and destruction of the desired texturing.Overall, RIE processes with fluorine and chlorine based plasmas canoffer excellent control over silicon etching directionality andselectivity to fabricate various desired silicon nanostructures.

Referring now to FIG. 13, the spectral response measurement forreflectivity demonstrates the potential of RIE texturing for reductionof light reflection from a surface. From FIG. 13, the UV-VISspectrophotometer results show that high aspect ratio (H.A.) nanotipstructures result in extremely low light reflection from the surface,and the weighted reflectance (R_(w)) of H.A. nanotip structures is only1.4% in the measured wavelength range (λ=300 nm-1100 nm). Table 1 belowlists the calculated weighted reflectance based on measured reflectancefrom FIG. 13. From these results, H.A nano-tip structures, nano-pillars,and dual-scale surface structures are observed to provide improvementsin reduction of light reflection from a surface.

TABLE 1 Wavelength (λ = 0.3-1.1 μm) R_(w) (%) Bare-Si 35.7 L.A. Nano-tip21.4 Nano-pillar 3.0 Dual-scale 2.0 H.A. Nano-tip 1.4

Regarding the application of RIE texturing processes to the manufactureof solar cells, the degree of surface plasma damage can be estimated anda damage recovery treatment can be performed on the Si surface.

Surface passivation also has the potential to further reduce surfacereflectance of RIE textured silicon surfaces. A Quinhydrone/Methanol(QHY/ME) passivation technique was applied textured samples in order toevaluate the textured surface for solar cell applications.

Three nominally identical p-type CZ with resistivity of 5-15 Ω-cm andthickness of 450 microns were tested. The samples were subjected tothree different surface treatments. Sample A was treated with HF andexposed to air before the measurement of effective carrier lifetime(τ_(eff)). Sample B was treated with an organic passivation (QHY/ME)layer on both sides, while sample C was RIE textured on one sidefollowed by treatment with an organic passivating layer on both sides.The organic passivating layer in other experiments (FZ, n-type <100>,100 Ω-cm) exhibited a high effective lifetime (˜3.2 ms) on FZ wafers,implying that the τ_(eff) for sample B in this experiment is controlledby the bulk lifetime. A τ_(eff) of 44 μs was measured for sample B, andcan be approximately considered equal to the bulk lifetime (τ_(bulk)) ofthe wafers. τ_(eff) for sample A was 4 μs, and this value approximatelycorresponds to wafers with infinite surface recombination velocity.τ_(eff) for sample C was 12 μs. Values for τ_(eff) were calculated usingequation 2:

$\begin{matrix}{\frac{1}{\tau_{eff}} = {\frac{1}{\tau_{b}} + \frac{S_{front}}{W} + \frac{S_{rear}}{W}}} & (2)\end{matrix}$

where S_(rear)=0 for a rear organic passivating layer, andS_(front)=2700 cm/s.

These results show that even the largest surface area of the types oftexturing can be passivated using passivation approaches such as organicpassivation. The number is likely to be an upper estimate of surfacerecombination using this the highly surface textured samples because thesubstrate was p-type, which has a weaker QHY/ME passivation effect.

In conclusion, RIE texturing with silica NS lithography can providegreat flexibility on the choice of the shape of texturing on surfacessuch as crystalline silicon. In one embodiment, by adjusting theparameters of RIE etching to control the etching properties on silica NSdeposited on a silicon surface, diverse shapes and textures wereproduced in the nano-scale (and submicron) range, which cansignificantly reduce the loss of incident sun light by reflection.Moreover, with the described solvent controlled spin-coating method,silica NS deposition on silicon surface is achieved in a cost-effectivemanner. Therefore, the combination of RIE texturing with silica NSlithography provides a cost-effective RIE surface texturing method forproducing myriad surface topographies.

For reactive ion etching (RIE) on SMS deposited Si surface, fluorine(F), and chlorine (Cl) based gas was used depending on the desired typeof etching (for example, isotropic/anisotropic). During the etchingprocess, gas flow rate, chamber pressure, RF power, and etching timewere varied to fabricate the desired shape of nanostructures bycontrolling the etching selectivity between SMS and Si and the etchingdirection (i.e., anisotropic, isotropic etching). More specifically,silicon substrates with silica NS monolayers were transferred to thechamber of a reactive ion etcher (PlasmaLab 80+, RIE). The RIE processwas set-up with a two or three step process depending on desired shape.However, processes involving less than two steps and more than threesteps are anticipated for achieving the desired surface texturing. ACHF₃/Ar gas blend was used for size control of silica NS and forstrip-off of the native oxide. Cl₂ gas was also used for anisotropicselective etching and SF₆/O₂ gas for isotropic selective etching betweenSiO₂ and silicon.

The etched surfaces were observed by scanning electron microscopy (SEM,JEOL XL-30), and surface reflectance was measured with a UV-VISspectrophotometer. The Sinton lifetime tester, WCT-120, was used foreffective carrier lifetime measurement after organic passivation on RIEtextured surface.

Although the etching steps of Example II to create dual scale texturingwith silica beads are described as being performed after the spincoating steps of Example I, it will be appreciated that the depositionaccording to Example I could be performed using techniques other thanspin coating. Accordingly, the dual scale texturing technique may beperformed on previously prepared substrates or substrates having silicabeads (and/or other materials) deposited on the substrate by othermeans.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

What is claimed is:
 1. A method of preparing a substrate surface, themethod comprising: (a) providing a material comprising an etching maskon a substrate; (b) subjecting the material to a first isotropic etchingphase; and (c) subjecting the material to a first anisotropic etchingphase, thereby forming a textured surface on the material, wherein thetextured surface comprises structures with dimensions in a sub-micronrange.
 2. The method of claim 1, wherein at least one of the firstisotropic etching phase and first anisotropic etching phase comprisesreactive ion etching.
 3. The method of claim 1, the method furthercomprising generating a plasma.
 4. The method of claim 1, wherein thestructures include nano-scale structures disposed on micro-scalestructures.
 5. The method of claim 1, further comprising subjecting thematerial to a second isotropic etching phase or a second anisotropicetching phase, or both.
 6. The method of claim 1, wherein the texturedsurface further comprises silicon nano-structures.
 7. The method ofclaim 1, wherein the textured surface further comprises spiked nano-rodstructures.
 8. The method of claim 1, wherein the textured surfacefurther comprises a substantially uniform array or a substantiallyordered array, or both.
 9. The method of claim 1, wherein the structuresare selected from the group consisting of rods, cones, frustums,pyramids and needles.
 10. The method of claim 1, wherein the texturedsurface is formed to exhibit a weighted reflectance of less than 5% overa wavelength range of about 300 nanometers to about 1100 nanometers. 11.A method of preparing a substrate surface, the method comprising: (a)dispersing a plurality of particles in a suspending medium to form asuspension; (b) spin-coating the suspension on a substrate comprising amaterial to form an etching mask on the substrate; (c) subjecting thematerial to a first isotropic etching phase, using the etching mask,forming a modified etching mask; and (d) subjecting the material to afirst anisotropic etching phase, using the modified etching mask,thereby forming a textured surface on the material, wherein the texturedsurface comprises structures with dimensions in a sub-micron range. 12.The method of claim 11, wherein the plurality of particles comprisescolloidal particles.
 13. The method of claim 11, wherein the suspendingmedium is selected from a group consisting of tetrahydrofuran, ethylacetate, acetone, dimethylformamide, acetonitrile, dimethyl sulfoxide,and mixtures thereof.
 14. The method of claim 11, the method furthercomprising generating a plasma.
 15. The method of claim 11, wherein thestructures include nano-scale structures disposed on micro-scalestructures.
 16. The method of claim 11, further comprising subjectingthe material to a second isotropic etching phase or a second anisotropicetching phase, or both.
 17. The method of claim 11, wherein the texturedsurface further comprises spiked nano-rod structures.
 18. The method ofclaim 11, wherein the textured surface further comprises a substantiallyuniform array or a substantially ordered array, or both.
 19. The methodof claim 11, wherein the structures are selected from the groupconsisting of rods, cones, frustums, pyramids and needles.
 20. Themethod of claim 11, wherein the textured surface is formed to exhibit aweighted reflectance of less than 5% over a wavelength range of about300 nanometers to about 1100 nanometers.